This invention relates to the field of hollow fibers used for gas separation, and to the art of sealing defects in such fibers.
It is known in the art that certain materials, which may be in the form of hollow fibers, are more permeable to some gasses than to others. The relative permeability of gasses through a material or article, e.g. a hollow fiber, is often expressed as a separation factor or ratio, which is equal to the flux of one gas divided by the flux of the other gas through the same material, measured under identical conditions of temperature and pressure.
Hollow fibers made of selective materials may be used in gas separation applications. These fibers typically are in the form of hollow tubular structures, either in a linear or bent configuration; gasses may freely pass through the open ends, but may not pass as easily through the fiber walls. The open ends provide access to the hollow fiber interior.
Generally in hollow fiber separations, gasses are separated when they are forced by a pressure differential to pass through or along the fiber. If the fiber is selective for one gas over another, the former gas will pass more easily, and therefore in larger proportion, through the wall of the fiber; the less permeable gas will have a lesser tendency to penetrate the fiber wall. In this way, the more permeable gas may be concentrated on one side of the fiber wall while the less permeable gas is concentrated on the other side. The gas concentrated in the fiber interior can be removed through an open end of the fiber.
However, gas separation will not occur efficiently if the fiber is defective, i.e. has a leak or hole in the wall, because every gas will be able to pass easily through the hole. Since a hole provides virtually no resistance to gas flow, much more gas is likely to pass therethrough than to pass through the intact parts of the fiber wall. For these reasons, a fiber defect can reduce significantly the separation factor of the fiber, often to the point of making the fiber useless for gas separation. Furthermore, when fibers are bundled together as in a gas separation module a small percentage of defective fibers may greatly reduce the separation efficiency of the bundle as a whole because the defective fibers will have a much higher gas throughput than will the non-defective fibers.
It is difficult to avoid fiber defects completely, especially when a large number of fibers are being used. Therefore it is important to find some way to plug the leaks in bad fibers without closing the open ends of good fibers. A number of solutions to this problem have been tried.
U.S. Pat. No. 3,499,062 describes a method for repairing leaks in a fluid separation apparatus containing hollow filaments in which a high pressure is applied to the interior fibers, a low pressure is applied to the exterior fiber surfaces, and a solidifiable liquid is applied to one end of the fibers under a pressure intermediate between the other pressures. According to this reference, this method causes the liquid to be drawn into and seal the leaky fibers. The method is repeated to seal the other end of the fibers.
U.S. Pat. No. 3,968,192 describes a method for repairing leaky hollow fiber permeability separatory devices comprising a membrane in the form of hollow fibers having ends potted in tubesheet bodies and terminating in an accessible surface of said tubesheet. Each surface opening communicating with a leak is selectively plugged by applying heat and pressure in the immediate vicinity thereof.
U.S. Pat. No. 4,226,921 describes a method for selectively plugging broken fibers in tubesheet-hollow fiber assemblies. The open ends are covered with a curable and flowable sealant and a pressure differential is used to draw the sealant into the fibers. The resistance to compression of the gas in the unbroken fibers keeps the sealant from penetrating too deeply. The sealant is then diluted and another pressure differential used to expel sealant from the unbroken fibers.
U.S. Pat. No. 4,248,648 describes a method for repairing leaks in a hollow capillary fiber diffusion device containing a bundle of hollow fibers. The fibers are temporarily sealed at one end and a curable sealant is applied to the other end. Suction is then applied to the fiber exterior, causing sealant to flow into bad fibers.